Normally tacky and pressure sensitive adhesive (PSA) materials have been used for well over half a century. Products of this type, which take the form of tapes, labels, and other types of adhesive coated sheets, must be protected from unintended adhesion to other surfaces. Hence, tapes are typically wound into a roll on their own backing and labels are typically laminated to a release sheet to prevent their accidental adhesion to other surfaces and also to prevent their contamination with air-borne dust and other contaminants. In order to allow a roll of tape to be unwound without the undesirable transfer of adhesive to the tape backing, it is customary to provide the tape backing with a low adhesion backsize (LAB). Similarly, the release sheet or liner, to which the adhesive coated label is typically laminated, is supplied with a release coating to permit the easy removal of the liner from the label. This LAB or release coating is expected to reproducibly provide an appropriate level of release to the adhesive of interest, to not deleteriously affect the adhesive, and to be resistant to aging so that the release level remains relatively stable with time.
The Handbook of Pressure Sensitive Adhesive Technology, 2nd Ed., D. Satas Ed., Van Nostrand Reinhold, New York, 1989, Chapter 23, describes polymers which may be used as release agents for PSA tapes. Various polymers of lower critical surface tension such as silicones, fluorine-containing polymers, and long alkyl chain branched polymers are useful as release coatings. Long alkyl chain branched polymers are waxy compounds which can be used to prepare release coatings of medium release value which are especially desirable for PSA tapes. Many release coating patents describe the use of such long alkyl chain branched polymers. For example, Hendricks, U.S. Pat. No. 2,607,711 (1952) discloses the use of copolymers of alkyl acrylate and acrylic acid for tape release coatings. According to Hendricks alkyl acrylates having an alkyl side chain of 16-20 carbon atoms are the more suitable, with octadecyl acrylate being the preferred comonomer.
Examples of other long alkyl chain branched copolymers include stearyl methacrylate-acrylonitrile copolymer (U.S. Pat. No. 3,502,497); copolymers of stearyl acrylate or methacrylate with other monomers (U.S. Pat. No. 4,241,198); polyvinyl esters such as polyvinyl stearate, polyvinyl palmitate, polyvinyl arachidate, and polyvinyl behenate (U.S. Pat. No. 2,829,073); stearyl maleate-vinyl acetate copolymer (U.S. Pat. No. 3,285,771); polyvinyl stearate, polyvinyl laurate, copolymers of vinyl stearate with vinyl acetate and maleic anhydride, copolymers of octadecyl acrylate with other monomers (U.S. Pat. No. 2,913,355); polyethylene imines a cylated with higher fatty acids (U.S. Pat. No. 3,510,342); poly-N-acyl imine (U.S. Pat. No. 3,475,196) ; solution polymers of vinyl stearate, allyl stearate, or vinyl octadecyl ether with maleic anhydride (U.S. Pat. No. 2,876,894); N-stearyl polyacrylamide (U.S. Pat. No. 3,051,588); solution polymerized stearyl itaconate, monacetyl itaconate, and monobehenyl itaconate (U.S. Pat. No. 3,052,566); copolymers of N-substituted long straight chain alkyl maleamic acids and vinyl monomers (U.S. Pat. No. 3,342,625); and polyvinyl N-octadecyl carbamate prepared by reacting polyvinyl alcohol and octadecyl isocyanate (U.S. Pat. No. 2,532,011).
Takaji, European Patent Application 80302935.4, Publication Number 0,024,908, Publication date Mar. 11, 1981, discloses polymeric release agents prepared via an organic solvent based polymerization of a vinyl monomer of the general formula CH.sub.2 .dbd.CR.sup.1 COO(CH.sub.2 CHR.sup.2 O).sub.n CONHR, wherein R.sup.1 and R.sup.2 each represent a hydrogen or methyl group, R represents an alkyl group of at least 12 carbon atoms or a fluoroalkyl group of at least 6 carbon atoms, and n is an integer of 1 to 6. European Patent Application 80302935.4 also discusses the polymerization of the vinyl monomer with other vinyl compounds. However, European Patent Application 80302935.4 does not teach or suggest the preparation of water-borne LABs or release coatings from such monomers.
All of the long-chain branched polymers discussed above are solvent-based polymers. They are prepared via polymerization reactions which are conducted in organic solvents and are typically coated out as dilute solutions, e.g., 1-10 percent, in the organic solvents in order to provide thin release coating layers on suitable backings. The use of such solvent-based release coatings in the manufacture of PSA tapes and other products poses both fire hazards and health hazards, in addition to being uneconomical, due to the organic solvents which constitute a large proportion of the coating solutions (i.e., generally greater than 90 weight percent). These organic solvents are typically either emitted into the air or burned in emission control devices. This is quite undesirable from the standpoint of environmental protection and energy conservation. An urgent need therefore exists for water-borne LABs and release coatings which can be prepared via aqueous polymerization reactions which can be coated onto suitable substrates as aqueous solutions or dispersions.
As indicated above, solvent-borne release coatings and LABs pose severe risks to the environment and are also uneconomical. Attempts have been made to overcome the problems associated with solvent-borne release coatings and LABs by the development of water-borne release coatings and LABs.
Collins, Great Britain Patent Specification 859,739 (published Jan. 25, 1961) discusses a backsize coating composition in the form of a substantially homogeneous and relatively stable aqueous dispersion adapted to be applied to a backing to form a thin coating which comprises a water-dispersible film former agent and a lesser amount of a water-insoluble water-dispersible release agent. The release agent which is incompatible with the film-former agent comprises either a fatty alcohol ester copolymer or fatty acid ester copolymer. Examples of the release agent include stearyl maleate-vinyl acetate copolymers and octadecyl acrylate-acrylic acid copolymers. The release agents are prepared by mixing the copolymers which are prepared and contained within organic solvents with isopropanol, water, and morpholine followed by distillation in order to remove the organic solvents. Thus, this approach is not a solvent-free process. In addition, the backsize coatings thus obtained do not provide good release properties for more aggressive pressure sensitive adhesives.
Dahlquist and Zenk, Great Britain Patent Specification 870022 (published Jun. 7, 1961) describes a pressure sensitive adhesive tape having a low adhesion backsize comprising a tetrapolymer of octadecyl acrylate, acrylic acid, acrylonitrile, and methyl acrylate in the approximate proportions by weight of 5:1:3:1. The tetrapolymer is obtained by the polymerization of the monomers in one or more organic solvents. Luedke, Dahlquist, and Hendricks, U.S. Pat. No. 3,011,988 (issued Dec. 5, 1961) discloses a method of converting such a tetrapolymer solution into an aqueous dispersion. The method involves the addition of water and morpholine to an organic solvent solution of the tetrapolymer followed by the removal of the organic solvent by distillation. Likewise, this approach is not a solvent-free process.
Doehnert, U.S. Pat. No. 4,299,741 (issued Nov. 10, 1981) discloses an aqueous release backsize coating composition for use in conjunction with pressure sensitive adhesive tapes. The coating composition comprises an acrylate terpolymer useful as a release polymer, a base, a polar solvent wetting agent, and a water-soluble thickening agent. The release polymer is obtained by the polymerization of a higher alkyl acrylate ester monomer, a carboxyl supplying monomer, and an acrylate derived hardening monomer in an organic solvent. A preferred acrylic ester is stearyl methacrylate. The aqueous backsize composition is prepared by adding a base to a solution of the terpolymer in an organic solvent, followed by the addition of a thickener and an aqueous solvent mixture. Again, this approach is not a solvent-free process. Furthermore, the backsize coatings prepared thereby do not provide the desired release or unwind properties for more aggressive pressure sensitive adhesives.
Wempe, U.S. Pat. No. 4,386,183 (issued May 31, 1983) and Wempe U.S. Pat. No. 4,440,830 (issued Apr. 3, 1984) disclose a polyvinyl alcohol based release coating composition for strippable substrates which is useful in conjunction with pressure sensitive adhesives. The release coating composition comprises a polyvinyl alcohol, a migratable release promoting agent, a water-soluble salt of a coordinating metal, and a water-soluble boron compound in an aqueous solution. According to Wempe the migratable release promoting agent may be any surface active agent exhibiting release promoting properties and surface activity in water and having a hydrophilic region which tends to bury itself in the water mass. Coatings formed from such compositions do not provide the desired release and aging properties for more aggressive pressure sensitive adhesives.
Thus, most existing polymer systems useful as LABs and release coatings which are described in the literature are solvent-based systems. However, due to safety, economical, and environmental concerns, solvent-free water-borne LAB and release coating systems are highly desirable. The so-called "aqueous release coating systems" described in the literature are mostly aqueous dispersions converted from solvent-based polymer systems and thus are not truly solvent-free polymer systems. In addition, such converted polymer systems do not provide desirable release and aging properties for more aggressive pressure sensitive adhesives. Other coating systems such as those that use surfactants as migratable release agents are also disadvantageous in that they do not provide desirable release and aging properties.
Attempts have been made at preparing aqueous-based dispersions of long alkyl chain branched polymers by the emulsion polymerization of monomer systems comprising long alkyl chain vinyl monomers such as octadecyl acrylate, stearyl methacrylate, vinyl stearate, etc. However, because of the relatively low reactivity of these long alkyl chain monomers as compared to short alkyl chain vinyl monomers in aqueous systems, difficulties have been encountered in attempting to incorporate these long alkyl chain vinyl monomers into a polymer structure. Thus, the polymer dispersions or emulsions prepared via such methods do not provide desirable release and aging properties. So far, there has been no successful development of a water-borne release coating system. An urgent need therefore exists for a water-borne release coating system which is environmentally safe and economical and which provides desirable release and aging properties.